Hi Denise,
Has this client given you any explanation as to why he/she is refusing to settle their final account and if so can the problem be ressolved? Is it only going to take putting a small matter right? Remember your business reputation is at stake here. I would not subject your staff to any further upset from this client .
Reading between the lines, this client is spending a lot of time trying to discredit your business which seems to suggest that your relationship has brokendown. As a result this could turn ugly and cause more harm than necessary. Remember the saying that 'Mud Sticks' and you are not entirely sure what is being said to others about you or your company.
If his reasons for not paying is unjust then might I suggest that you return these keys and depending on the amount owed let him/her know that you intend to proceed with legal remedies by bringing a small claims action against him. To hold onto these keys serves no purpose because to change locks to the property is an option they may want to take and a cheaper one at that.
The value of this property far out weights the cost of the amount owed to you. By an action to recover the debt may solve this problem especially when it is going to affect your clients credit status. Stay positive & professional and send your client a letter stating what you intend to do if this account is not settled within the next 7 days.
This need not be a long drawn out process and can purely be administrative using the right process at the courts. Meaning you could see a turn around within 4-6weeks for an initial outlay of approx £120. It may seem expensive but if your client does not want to fall foul of being entered onto a black list for future creditors it may force them to pay up quickly.
I not sure whether you have a written agreement and the terms contained within or how long you have serviced this client; nonetheless you have an agreement whether this be in written or verbal form.
It is hard to really advise without knowing all the facts in contention but on the face of the facts you present try the above course of action.
Good luck!!